A distributed switching system is one comprised of a plurality of switch units which interface customer lines and trunks on a peripheral side of the unit and which interface a central switch arrangement on the other side of the unit. Calls between customers and/or trunks connected to different switch units are completed through the central switch. The AT&T system disclosed in the AT&T Technical Journal, July-August 1985, No. 6, Part Two, is a time division switching system where the switching function is distributed to a plurality of switching modules (SMs), each connected to a number of lines and/or trunks. Each SM provides connections among the lines and trunks connected to that module. Calls involving lines or trunks connected to different SMs are completed through a time-multiplexed switch (TMS) that interconnects the SMs. Each SM includes a control unit that controls the switching function of that SM. The system also includes a central control that controls the switching function of the TMS. All calls within the system require the selection of what is referred to as a network time slot. For inter-module calls, the network time slot is used for transmission from one SM, through the TMS, to another SM. The same network time slot is used for both directions of transmission. For intra-module calls, the network time slot is used within the SM to connect one line or trunk to another line or trunk. Two network time slots are used for intra-module calls, one for each transmission direction. Although the call processing function is distributed in the system in that the real-time intensive tasks associated with calls, e.g., signal processing, are performed by the switching module control units, the functions of selecting the network time slot and setting up the TMS path if the call is an inter-module call, are centralized, being performed by the central control. There are 512 channels (time slots) TS0 through TS511 between a given SM and the TMS. Setting up a path for an inter-module call between two SMs involves finding a channel that is available on the link to the first SM, for example TS44, and that has a corresponding available channel TS44 on the link to the second SM. The central control stores an availability bit map for each of the links to the SMs for use in performing the network time slot selection function. Even though no TMS path is required for an intra-module call, the two network time slots assigned to the call are not otherwise usable. Therefore the network time selection (or path hunt) function is performed by the central control for both intra-module calls and inter-module calls.
One of the important advantages of a modular system of this type is that its capacity can be closely matched to the requirements of specific applications. However, as the system becomes larger, the performance of per call tasks, such as network time slot selection, by a central control for all calls, imposes an upper limit on the overall system call processing capacity.